1. Field of the Invention
The present invention relates to a sheet supplying apparatus used with an image forming apparatus such as a laser beam printer, an ink jet printer, a copying machine, a facsimile apparatus and the like.
2. Related Background Art
In the past, there has been proposed a sheet supplying apparatus wherein a sheet stacking plate on which a plurality of sheets are stacked is lifted and lowered between a waiting position and a supply position, and an uppermost sheet is feed out by a sheet supply roller at the supply position. In some of such sheet supplying apparatuses, the sheet stacking plate is biased toward the supply position by a spring and the like, and the sheet stacking plate is lowered to the waiting position in opposition to the biasing force of the spring by a cam secured to a shaft of the sheet supply roller when this roller is rotated.
In such apparatus, in an operative condition, whenever the sheet is supplied one by one, the sheet stacking plate is shifted from the supply position to the waiting position, and in an inoperative condition, the sheet stacking plate is held in the waiting position to facilitate the replenishment of sheets or the like.
Generally, the sheet stacking plate is held in the waiting position by a means for interrupting the transmission of a rotational force to the shaft of the sheet supply roller, and a stopper capable of stopping the sheet stacking plate at the lowered position. By releasing the stopping action of the stopper and the interruption of the transmission of the rotational force to the sheet supply roller, the cam is rotated to shift the sheet stacking plate from the waiting position to the supply position, thereby permitting the sheet supplying operation.
The concrete construction will be described hereinbelow.
In FIG. 28, a plurality of sheets S are stacked on a sheet stacking plate 151 which is biased by a spring 153 so that the sheet stack can be urged against a sheet supply roller 152 for supplying a sheet S. Further, a friction member 154 as a separation means disposed in front of a front end of the sheet stacking plate 151 is biased by a spring 155 so that the friction member can be abutted against the sheet supply roller 152. The sheets S supplied from the sheet stacking plate 151 to the sheet supply roller 152 are separated one by one by the friction member 154.
A cam 156 is secured to a shaft 152a of the sheet supply roller 152, which cam can be abutted against the sheet stacking plate 151. The cam 156 is rotated as the sheet supply roller 152 is rotated, thereby shifting the sheet stacking plate between a waiting position and a supply position. That is to say, the sheet stacking plate 151 can be shifted between the waiting position (condition shown in FIG. 28) where the sheet stacking plate 151 is lowered by the cam 156 in opposition to the biasing force of the spring 153 and the supply position where the cam 156 is separated from the sheet stacking plate 151 and the sheet stack is urged against the sheet supply roller 152.
A rotational force of a motor M is transmitted to the sheet supply roller 152 by a gear 157 secured to the roller shaft 152a and a gear 158 meshed with the gear 157 and connected to the motor M. The gear 157 has a cut-out portion 157a. When the cut-out portion 157a faces the gear 158, the rotational force is not transmitted to the sheet supply roller.
Further, the cam is subjected to a force from the sheet stacking plate 151 by the spring 153 for biasing the sheet stacking plate 151 so that the cam is rotated in a clockwise direction in FIG. 28. However, the cam can be held in the waiting position by a stopper 159. The stopper 159 comprises a regulating member 161 engaged by a regulating cam 160 and adapted to regulate the rotation of the roller shaft 152a, and an electromagnet 162 for shifting the regulating member 161 between a regulating position and a non-regulating position.
The operation of this apparatus will be briefly explained hereinbelow.
FIG. 28 shows the waiting condition of the sheet stacking plate 151. When a signal such as a sheet supply signal is inputted to the apparatus, the electromagnet 162 is operated to disengage the regulating member 161 from the regulating cam 160. As a result, the cam 156 is rotated in the clockwise direction in FIG. 28 by the spring 153 via the sheet stacking plate 151. Further, in the waiting condition, since the cut-out portion 157a of the gear 157 faces the gear 158, the rotational force of the motor M is not transmitted to the sheet supply roller 152. However, when the cam is rotated by releasing the regulating member 161, the gear 157 is engaged by the gear 158, with the result that the rotational force is transmitted to the sheet supply roller 152. The sheet stacking plate 151 is lifted to urge the sheet stack S against the sheet supply roller 152, thereby feeding out the sheet.
When the cam 156 is rotated by one revolution, since the cut-out portion 157a of the gear 157 faces the gear 158 again, the transmission of the rotational force is interrupted. In this case, when the electromagnet 162 is in an inoperative condition, the cam 156 is stopped by the regulating member 161, thereby bringing the sheet stacking plate 151 to the waiting condition shown in FIG. 28.
Incidentally, in the above example, while the transmission of the rotational force is controlled by the combination of the gear 157 having the cut-out portion and the gear 158, a spring clutch for effecting one-revolution control may be used.
The spring clutch is conventionally used and is provided on the drive shaft of the sheet supply roller, so that the rotational force of the motor is transmitted to or interrupted from the drive shaft by tightening or loosening a coil spring of the spring clutch.
However, in the above-mentioned sheet supplying apparatus has the following disadvantages.
In the case where the transmission of the rotational force is controlled by the combination of the gear 157 having the cut-out portion and the gear 158, if the rotational speed of the gear 157 is faster than the rotational speed of the gear 158 when the gear 157 is engaged by the gear 158 after the idle rotation of the cut-out portion 157a in consequence of the clockwise rotation of the cam 156 by the spring 153 for biasing the sheet stacking plate 151, the great torque will be transmitted from the gear 157 to the motor M, thus affecting a bad influence upon the motor M, or making the feeding speed of a convey roller 163 unstable due to fluctuation in the rotational speed of the motor M caused by the variation of torque when the convey roller 163 is driven by the same motor M.
Further, in the case where the spring clutch is used, since the sheet stacking plate 151 biases the cam 156 thereby to push the roller shaft 152a toward the rotation direction of the sheet supply roller 152, the spring clutch is loosened or untightened, and, therefore, the rotational speed of the shaft 152a becomes greater than the rotational speed of the motor accordingly. This occurs acceleratively, thereby increasing the lifting speed of the sheet stacking plate 151, with the result that the sheet stack S rested on the sheet stacking plate 151 is struck against the sheet supply roller 152, thereby applying the great fluctuation of load to the motor M or generating the great striking noise.